Underlay Calculator
Enter the floor area and coverage printed on the underlay roll to estimate a whole-roll order. The tape estimate assumes parallel strips; check the actual layout and the chosen product’s joint instructions before buying.
Your measurements
Check your order
Your flooring order
- Confirm the layout, cuts, pack coverage and allowance before rounding to whole packs.
- Check whether the flooring needs separate underlay and which moisture-control system is specified.
- List edge trims and doorway transitions separately; check the permitted installation tools.
Tools for this job
Check room dimensions and roll direction before cutting. A utility knife suits some foam underlays; other materials require different cutters.
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- Score and trim suitable sheet materials
Utility knife
STANLEY Classic 99 retractable utility knives
For scoring or trimming sheet materials that allow knife cutting. Choose a blade suited to the material and support the work on a cutting surface.
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Short tape measure
STANLEY 25-foot tape measure
For checking lengths, widths and short runs before ordering. This is a 25-foot tape; use matching units in the calculator and check the scale on the selected model.
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Already have suitable tools? Reuse them. These are optional product suggestions, not a complete equipment list or a hands-on comparison. Amazon may redirect Amazon.com links to local products or search results. Amazon.nl links search for alternatives, which may differ in brand, size and specification. Check suitability, seller, delivery and any import costs on your chosen store before ordering.
How much underlay do I need?
Underlay is a straightforward area calculation: floor area plus a small allowance, divided by the coverage on the roll. The allowance covers trimming at the walls and the small amount lost where strips meet an alcove or doorway.
The selected allowance is for cutting and trimming, not permission to stack cushioning layers. Check how the specified system joins its underlay and moisture-control layer. An integrated membrane flap can have a different joint detail from the cushioning material.
The formula
Check compatibility before choosing a roll
| Project detail | What to record | Why area alone is insufficient |
|---|---|---|
| Flooring system | Exact model and permitted underlay | Thickness alone does not establish support or locking-system compatibility |
| Attached pad | Whether extra underlay is allowed | A second layer is not automatically suitable |
| Subfloor | Required flatness, moisture condition and preparation | Underlay coverage does not prove the substrate is ready |
| Moisture control | Separate membrane or integrated layer; joint and edge details | Tape and flap requirements can differ between products |
| Underfloor heating | Permitted combined thermal resistance and heating instructions | Check the complete floor assembly rather than a generic thickness |
| Acoustic requirement | Required system performance and tested assembly | A material description is not an acoustic rating |
Worked example
A 5 m x 4 m room = 20 m². With a 7% allowance that is 21.4 m². At 15 m² per roll you need 1.43 rolls → 2 rolls, covering 30 m². The 10 m² above the room area includes the cutting allowance; it is not a promise of 10 m² of reusable offcuts. A 10 m² roll would require three rolls for the same estimate, so compare the full order price.
Assume 1 m wide strips run along the 5 m room length. Four strips create three lengthwise joints of 5 m: 15 m of tape, or 16.5 m with the calculator’s 10% tape allowance. This excludes end joints and perimeter details and assumes the product needs tape on those joints.
Installation direction and heating requirements
Follow the selected system’s laying direction. For example, Quick-Step’s laminate instructions say to lay its underlay in the same direction as the floor and describe the membrane flap and joint sealing. Do not apply a universal “always at right angles” rule to every product.
For underfloor heating, check the flooring, underlay and heating instructions together. Record the allowed combined thermal resistance, temperature limits and preparation requirements. A generic tog limit or nominal thickness cannot establish that the selected assembly is permitted. The linked Quick-Step page also provides its heating instructions.
Mistakes that cost you a roll
- Confusing a membrane overlap with overlapping or doubling the cushioning layer. Follow the specified joint detail.
- Forgetting the doorway and the area under a radiator.
- Buying underlay by thickness or acoustic description without checking it is permitted beneath the chosen flooring.
- Leaving required moisture-control materials, tape or perimeter details out of the order.
Sources and assumptions
Roll quantity uses floor area plus a cutting allowance divided by roll coverage. Tape length estimates the seams between strips, with an extra allowance.
- Quick-Step: installing an underlay: The manufacturer describes trimming underlay to the room and avoiding stacked layers that create an uneven floor.
Before using this estimate
- Area-only mode assumes a square room when estimating strip and tape lengths. Enter room dimensions if the floor is a long hallway.
- The estimate does not establish moisture resistance, thermal resistance or compatibility with the floor. Follow the chosen system for seam treatment, membrane flaps and laying direction.
Source notes reviewed . See how we check calculations and handle corrections.
Planning your order
Check the inputs before buying: measure the surfaces you need, then apply an allowance and round to whole packs. The guides include worked examples you can follow alongside this calculator.
Published by OrderEnough. This is a planning estimate: check the product specification and the assumptions above before ordering. Last updated . Read how we source our figures on the methodology page.